| 1. |
- Abreu, P., et al.
(författare)
-
Study of dimuon production in photon-photon collisions and measurement of QED photon structure functions at LEP
- 2001
-
Ingår i: European Physical Journal C. - : Springer Science and Business Media LLC. - 1434-6044 .- 1434-6052. ; 19:1, s. 15-28
-
Tidskriftsartikel (refereegranskat)abstract
- Muon pair production in the process e+e- → e+e- μ+μ- is studied using the data taken at LEP1 (√s ≃ mz) with the DELPHI detector during the years 1992-1995. The corresponding integrated luminosity is 138.5 pb-1. The QED predictions have been tested over the whole Q2 range accessible at LEP1 (from several GeV2/c4 to several hundred GeV2/c4) by comparing experimental distributions with distributions resulting from Monte Carlo simulations using various generators. Selected events are used to extract the leptonic photon structure function Fγ 2. Azimuthal correlations are used to obtain information on additional structure functions, Fγ A and Fγ B, which originate from interference terms of the scattering amplitudes. The measured ratios Fγ A/Fγ 2 and FγB/Fγ 2 are significantly different from zero and consistent with QED predictions.
|
|
| 2. |
|
|
| 3. |
- Bergmann, U, et al.
(författare)
-
High-resolution X-ray spectroscopy of rare events : a different look at local structure and chemistry
- 2001
-
Ingår i: Journal of Synchrotron Radiation. - Univ Calif Berkeley, Lawrence Berkeley Lab, Phys Biosci Div, Berkeley, CA 94720 USA. Univ Calif Davis, Dept Appl Sci, Davis, CA 95616 USA. CALTECH, Jet Prop Lab, Pasadena, CA 91109 USA. : MUNKSGAARD INT PUBL LTD. - 0909-0495 .- 1600-5775. ; 8, s. 199-203
-
Tidskriftsartikel (refereegranskat)abstract
- The combination of large-acceptance high-resolution X-ray optics with bright synchrotron sources permits quantitative analysis of rare events such as X-ray fluorescence from very dilute systems, weak fluorescence transitions or X-ray Raman scattering. Transition-metal K beta fluorescence contains information about spin and oxidation state; examples of the characterization of the Mn oxidation states in the oxygen-evolving complex of photosystem II and Mn-consuming spores from the marine bacillus SG-1 are presented. Weaker features of the K beta spectrum resulting from valence-level and 'interatomic' ligand to metal transitions contain detailed information on the ligand-atom type, distance and orientation. Applications of this spectral region to characterize the local structure of model compounds are presented. X-ray Raman scattering (XRS) is an extremely rare event, but also represents a unique technique to obtain bulk-sensitive low-energy (<600 eV) X-ray absorption fine structure (XAFS) spectra using hard ( 10 keV) X-rays. A photon is inelastically scattered, losing part of its energy to promote an electron into an unoccupied level. In many cases, the cross section is proportional to that of the corresponding absorption process yielding the same X-ray absorption near-edge structure (XANES) and extended X-ray absorption fine structure (EXAFS) features. XRS finds application for systems that defy XAFS analysis at low energies, e.g. liquids or highly concentrated complex systems, reactive compounds and samples under extreme conditions (pressure, temperature). Recent results are discussed.
|
|
| 4. |
|
|
| 5. |
|
|
| 6. |
- Messinger, Johannes, 1963-, et al.
(författare)
-
Absence of Mn-centered oxidation in the S2 → S3 Transition: : Implications for the mechanism of photosynthetic water oxidation
- 2001
-
Ingår i: Journal of the American Chemical Society. - : AMER CHEMICAL SOC. - 0002-7863 .- 1520-5126. ; 123:32, s. 7804-7820
-
Tidskriftsartikel (refereegranskat)abstract
- A key question for the understanding of photosynthetic water oxidation is whether the four oxidizing equivalents necessary to oxidize water to dioxygen are accumulated on the four Mn ions of the oxygen-evolving complex (OEC), or whether some ligand-centered oxidations take place before the formation and release of dioxygen during the S-3 --> [S-4] --> So transition. Progress in instrumentation and flash sample preparation allowed us to apply Mn K beta X-ray emission spectroscopy (K beta XES) to this problem for the first time. The K beta XES results, in combination with Mn X-ray absorption near-edge structure (XANES) and electron paramagnetic resonance (EPR) data obtained from the same set of samples, show that the S-2 --> S3 transition, in contrast to the S-0 --> S-1 and S-1 --> S-2 transitions. does not involve a Mn-centered oxidation. On the basis of new structural data from the S-3-state, manganese mu -oxo bridge radical formation is proposed for the S-2 --> S-3 transition, and three possible mechanisms for the O-O bond formation are presented.
|
|
